Several types of containers are (at least allegedly) specifically manufactured to withstand the mechanical stresses involved by the hot filling and the subsequent changes of internal pressure due to the temperature drop.
It is known to provide the container sidewall with flexible pressure panels the curvature of which changes to compensate for the change of pressure inside the container, as disclosed in European Patent No. EP 0 784 569 (Continental PET). One main drawback of this type of container, however, is its lack of rigidity once opened. Indeed, the pressure panels tend to bend under the grabbing force of the user, who should hence handle the container with care to avoid unintentional splashes.
It is also known to provide the container with a rigid sidewall and a flexible base including an invertible pressure panel.
In a first technique, the pressure panel is flexible and self adjusts to the changes in pressure inside the container. U.S. Pat. No. 8,444,002 (Graham Packaging) discloses a container the base of which is provided with a pressure compensating panel having numerous hinges and panels, which progressively yield or yield simultaneously depending on the pressure difference between the inside of the container and the outside of the container. Although such a structure has proved efficient to adapt to the changes in pressure inside the container and to maintain the shape of the container sidewall when the container stands alone, it does not provide the necessary strength to withstand external stresses such as vertical compression stresses undergone by the container when stacked or palletized.
In a second technique, disclosed in U.S. Pat. Appl. No. 2008/0047964 (Denner et al, assigned to CO2PAC), in order to alleviate all or a portion of the vacuum forces within the container, the pressure panel is moved from an outwardly-inclined position to an inwardly-inclined position by a mechanical pusher after the container has been capped and cooled, in order to force the pressure panel into the inwardly-inclined position.
Tests conducted on such a container showed that, once inverted to the inwardly-inclined position, the pressure panel does not maintain its position but tends to sink back under the pressure of the content. In the end, after the content has cooled, the container has lost much rigidity and therefore feels soft when held in hand. When stacking or palletizing the containers, there is a risk for the lower containers to bend under the weight of upper containers, and hence a risk for the whole pallet to collapse.